The present invention generally relates to a method for controlling a digital telephone set and a telephone set based on the same, and particularly to a method for controlling a digital telephone set which uses an integrated services digital network as a transmission line and a telephone set based on the method.
Recently, there has been considerable activity in the development of an integrated services digital network (hereinafter simply referred to as an ISDN). Currently, terminal equipments for an ISDN, such as a digital telephone set have been put to practical use. An ISDN will take place of a public services telephone network (hereinafter simply referred to as a PSTN) in the feature. However, both ISDN and PSTN will be used until an ISDN is completely substituted for the PSTN. In order to construct a network including both the ISDN and PSTN, internetworking has been proposed, which enables it to be possible to exchange data between the ISDN and PSTN.
FIG.1 illustrates a basic transmission control procedure which is to be executed when an ISDN is used. In the illustrated procedure, a digital telephone set is used. An operator related to a caller (source) terminal takes off a handset of a digital telephone set, and inputs the telephone number of a destination terminal. Then the source terminal sends a call set-up message SETUP to an ISDN to thereby request to set up a call for the destination terminal. The ISDN sends the designated destination terminal the call set-up message SETUP. Thereby, the destination terminal is called out. Further, the ISDN sends the source terminal a call set-up accepted message CALL PROC used for letting the source terminal know a call set-up state.
When the destination terminal receives the call set-up message SETUP, it starts ringing a buzzer in order to let an operator related to the destination terminal know the arrival of a call. At the same time, the destination terminal sends the ISDN a call message ALERT which lets the source terminal know that the destination terminal has started ringing. Thereby, the source terminal starts outputting a calling indication by outputting a ringing tone through the handset, for example.
When the operator related to the destination terminal takes off the handset, the destination terminal sends the ISDN an answer message or connection message CONN. Correspondingly, the ISDN sends the source terminal the answer message CONN to thereby let the source terminal know that the destination terminal accepts the call. Thereby, the source terminal stops outputting the calling indication.
The ISDN sends the destination terminal an answer confirmation message or a connection acknowledgement CONN ACK. At this time, an information channel for data transfer is established between the source and destination terminals. Thereby, the operators of the source and destination terminals can communicate with each other.
When speech ends and the operator related to the source terminal puts back the handset, for example, the source terminal sends the ISDN a disconnection message DISC in order to request a release of the information channel. The ISDN sends the destination terminal the disconnection message DISC for letting the destination terminal know the release of the information channel. Thereby, the destination terminal sends back the ISDN a release message REL for letting the source terminal know that the channel disconnection is completed. The ISDN sends the source terminal the release message REL. At this time, the destination terminal outputs an indication showing the end of speech through a display unit provided in the destination digital telephone set in order to let the operator know that the information channel is disconnected.
When the channel release is completed, the source terminal sends the ISDN a release completion message REL COMP. Then, the ISDN sends the destination terminal the release completion message REL COMP, and then the release of the information channel is completed. Hence, the information channel established between the source and destination terminals is completely released, and the line returns to the idle state. The line disconnection may be requested by the destination terminal.
In this manner, the information channel is established between the source and destination terminals, and is then released from the connection state.
It is noted that there is proposed an ISDN which accepts all the digits of the destination telephone number together at one time in order to set up a call. The call set-up procedure related to this case is especially called a batch call set-up procedure. An ISDN based on this procedure cannot operate when the digits of the destination telephone number are sequentially supplied thereto. Conventionally, when a call is set up for an ISDN based on the batch call set-up procedure, the operator related to a source terminal inputs all the digits of the destination telephone number, and thereafter gives the source terminal a specific instruction for having the source terminal start the call set-up operation.
On the other hand, in the conventional PSTN, the operator related to a source terminal (an analog telephone set) inputs the destination telephone number for every digit when setting up a call. Therefore, two different call set-up procedures must be provided, and suitable one of the procedures must be selected depending on the type of the connected ISDN. This is very troublesome.